This invention relates to an matrix impact printer. It relates more particularly to an improved print head for such a printer.
An impact printer is one which has a plurality of print elements which can be extended selectively to impact against paper or other web carried on a platen. A web having a transfer coating, such as carbon ribbon, is interposed between the print head and the paper so that when a printing element impacts against the ribbon, a character is printed onto the paper. The most obvious example of an impact printer is a conventional typewriter each of whose printing elements carries an embossed character which is printed onto the paper when that printing element is actuated. Thus a separate printing element is required for each character to be printed.
In a matrix printer, the characters are composed of tiny appropriately positioned dots. These dots are formed by a print head having relatively few identical print elements in the form of thin wires whose corresponding ends are positioned at the working end of the head opposite the platen. The formation of a given character involves alternately extending selected ones of the wires toward the platen and displacing the print head and platen relative to one another. Thus, for example, in a typical matrix printer, the print head may contain a vertical column of seven wires and be movable horizontally relative to the platen five steps for each character location. In other words, each character location may be considered as a 5.times.7 grid or matrix, with the identity of the character at that location being determined by which of the seven wires in the vertical column are actuated at each of the five horizontal locations in the grid. Since the printing elements used in the print head of a matrix printer are small and lightweight, they have relatively low inertia so that the printer can print at a high rate of speed. As such, matrix printers are often used to print out data from high-speed computers.
In one conventional type of print head of primary interest here, a set of print wires are slidably mounted in the print head along a general direction parallel to the axis of the head. Corresponding ends of those wires are aligned in a vertical column at the working end of the head which is positioned opposite the platen and the usual carbon ribbon and paper are trained between the head and the platen. The opposite ends of the wires, each terminating in an anvil, are distributed relative to the head axis at the opposite end of the head. Associated with each print wire is a solenoid actuator, the actuators being positioned to strike the associated print wire anvils.
The actuators employed in the prior printers of this general type usually comprise an electromagnet or solenoid oriented parallel to the print head axis and a striker pivotally mounted opposite the solenoid and with its end disposed opposite the anvil on an adjacent print wire. When the solenoid is energized, it attracts its striker which thereupon strikes the associated print wire anvil displacing that wire along the print head axis. Thus the print wire is extended momentarily from the working end of the print head and impacts the ribbon and paper to produce a printed dot on the paper.
Each actuator also invariably includes a separate spring for biasing its striker away from its solenoid so that the striker is assured of traveling a sufficient distance to properly impulse the print wire each time the solenoid is energized. Also, known printers of this type provide a separate small coil return spring for each print wire to assure that each wire retracts promptly and completely into the print head following each actuation thereof.
While the prior print heads employing such print wires and actuators are widely used, they are not as efficient as they might be. It is believed that this is because their actuators are characterized by relatively poor magnetic performance and high inertia. Consequently, a relatively large amount of power is required to drive each actuator so that it overcomes its spring bias and causes the associated print wire to impact the paper with sufficient force to print a distinct dot on a reliable basis. Such high power utilization is not only reflected in higher operating cost for the prior printers, but also a considerable amount of that power is dissipated as heat in the print head. That heat adversely affects the components of the head thereby increasing head maintenance costs and shortening the service life of the head.
Some prior heads are also adversely affected because repeated actuations of the print wires cause undue wear of the wire anvils as well as the wire guides that serve to locate the wires in the head. The former problem is due to the repeated impacts of the strikers against the separate anvils. It is believed that the latter wear problem is due to inappropriate arrangements of print wire actuators and wire guides that produce undue bending of the wires along their courses to the working end of the head. Moreover, the print wires in some prior heads are prone to jam because paper and dirt particles invariably accumulate in the head and the separate small coil return springs are unable to overcome the resistance presented by such debris and properly retract the wires. All of these factors further increase head maintenance problems.
Finally, the prior heads are relatively complex and difficult to assemble so that they are expensive to make.